SEIDS runs into a common Logic Pro glitch: hit playback in the middle of a recorded MIDI chord and nothing sounds, even though notes are clearly held down underneath the playhead.
The fix lives in Project Settings (not Preferences), under MIDI Chase. Turning on the Notes option tells Logic to catch any notes already sounding when playback starts.
Once enabled, starting mid-chord plays the full harmony instead of silence, a small setting that saves real confusion when editing or looping through MIDI parts.